Optical absorption tails and structural disorder in ${\mathrm{Sn}}_{\mathrm{x}}$${\mathrm{Ge}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Se}}_{2.5}$ and other chalcogenide alloy glasses and liquids

The Urbach slopes σ of optical absorption edges and the glass transition temperatures $T_g$ in pseudobinary chalcogenide alloy glasses with 0≤x≤0.6 containing predominantly tetrahedral building blocks have been studied as the resistance to shear of the building blocks is varied through the critical composition x=0.4. The composition dependences of σ and $T_g$ are nearly parallel, with softening occurring above x=0.4, in agreement with theoretical expectations. Chemical trends in Urbach slopes of other chalcogenide alloy glasses and liquids are discussed within the framework of constraint theory and a simple statistical model.